1. Field of the Invention
The present invention relates to electrochemical cells generating electrical energy by means of a chemical reaction. Electrolytic cells, for example of the lithium/silver vanadium oxide (Li/SVO) type, are typically constructed of one or more layers of anode, separator, and cathode. A screen or foil current collector is enclosed in the anode and cathode to transport electrons. An electrode assembly may be built by stacking multiple layers or plates on top of each other or by winding one or more long strips of the stacked layers around a mandrel. The electrode assembly is placed inside a case and immersed in an electrolyte, which transports ions.
One of the concerns in constructing an electrochemical cell is ensuring that the anode and cathode electrodes are properly aligned. This is not as great a problem is jellyroll electrode assemblies where the electrodes are of plates that are substantially longer than they are wide. The electrodes are then laid one on top of the other and spirally wound into the jellyroll configuration.
However, in an electrochemical cell having a multi-plate construction, electrode misalignment is a concern. Misalignment results in there being electrode plates that are not directly opposed by plates of an opposite polarity. In that respect, electrode plate misalignment detracts from the cell's discharge efficiency, as there will be active material that may not be fully reacted during electrochemical discharge. This is particularly likely to occur at the electrode edges.
The present invention prevents such misalignment by providing at least one of the electrode current collectors with projections emanating from its corners. These protrusions help to precisely position the current collector in a pressing fixture for contacting an active material to both sides thereof. That way, active material is contacted to each of the major faces of the current collector and is of a uniform thickness about its edges. Later, when the electrode plate is assembled into an electrochemical cell, such as of a multi-plate construction, the protrusions also serve to maintain strict alignment of the plate inside the casing.
Without protrusions according to the present invention, it is possible for the current collector to be positioned inside a pressing fixture with one portion of its edge too close to the fixture sidewall and another portion positioned too far away from the fixture. The result is that there is too much active material at the current collector edge spaced from the fixture sidewall and not enough at the other edge. This unbalanced active material contact can result in diminished discharge efficiency when the plate is incorporated into an electrochemical cell.
2. Prior Art
U.S. Pat. Nos. 627,134 to McDougall and 1,600,083 to Webster relate to current collectors having apertured projections. The projections do not contact the casing sidewall to ensure proper alignment. Instead, they receive locking rods for maintaining alignment inside a battery. Also, the prior art projections are not capable of centering the current collector in a pressing fixture. For example, with a generally rectangular shaped current collector, the centering projections must emanate from the corners at about a 45° angle, or essentially centered between the two contiguous sides. That way, with the current collector positioned in a fixture having the protrusion nested in a fixture corner, the immediately adjacent current collector sides are spaced from the pressing fixture sidewall by a like distance. The prior art current collectors do not provide for this type of centering as their protrusions emanate from a current collector edge adjacent to a corner. A corner emanating protrusion provides for proper spacing along the current collector edge having the protrusion, but not along the adjacent edge.
An example of this is shown with the current collector 10 illustrated in FIG. 1. The current collector 10 comprises first and second major faces 12 and 14 extending to a surrounding perimeter edge formed by opposed right and left edges 16 and 18 extending to upper and lower edges 20 and 22. The right and left edges 16, 18 and the upper edge 20 are straight while the lower edge 22 is curved. The current collector 10 has an interior perforated region 24. Spaced apart protrusions 26 and 28 emanate from the upper edge 20 adjacent to the respective right and left edges 16 and 18. Similarly, spaced apart protrusions 30 and 32 emanate from the curved edge 22 adjacent to the respective right and left edges 16 and 18. Having a protrusion only emanating from one edge of a current collector, instead of a corner between adjacent edges, means that there is no structure for regulating the spacing of the other current collector edge within a pressing fixture or a casing sidewall, as the case may be. In other words, protrusion 26 correctly spaced the upper edge 20 from a fixture sidewall (not shown), but is incapable of regulating the distance between the fixture and the right edge 16 of the current collector 10. A similar problem exists with respect to protrusion 30 and edge 16 and protrusions 28 and 32 and edge 18.
Thus, there is a need for a current collector design that enhances alignment in a pressing fixture so that a desired thickness of active material contacts both major current collector faces and the surrounding edge. Additionally, the current collector must provide for proper alignment with the opposite polarity electrode when it is incorporated into an electrode assembly housed inside a cell casing. The present current collector design provides both of these benefits.